Displays are used in a variety of applications. They serve for providing images, i.e. visual information, to a user. The quality of these images is a key issue for the user, i.e. the viewer or observer. Image quality flaws, for instance image noise, blurring, flicker, lack of spatial resolution or unnatural colours, deteriorate the performance. This is not only a problem affecting the user's convenience, which is of course exceedingly important as a selling point, but also constricts the scenarios of use of a display.
For example, a display employed in the field of medical technology providing only poor image quality may be a cause of severe harm to a patient if either still image content or video content is not reproduced authentically.
When developing display technologies, characteristics of human visual perception have to be considered.
In principle, light entering the eye is projected onto the retina. The retina comprises different types of cells, namely cone-cells and rod-cells. These cells convert light into neural signals that are transmitted to the brain via the optic nerve. There, the neural signals are processed to create an image.
Rod-cells are much more light-sensitive than cone-cells. They allow for visual perception even in dark surroundings. Cone-cells are less light sensitive and are responsible for colour perception. Colour is an important characteristic of visual information.
The human eye is provided with three different types of cones, each having its peak of light-sensitivity at a different wavelength. In particular, a first type of cones is sensitive to blue light, a second type is sensitive to green light and a third type is sensitive to red light. The brain merges the neural signals of all three types of cones to create a full colour image. Thus, humans possess three independent channels for perceiving colour information. Human vision is therefore called trichromatic.
Human visual perception is highly developed. It allows for the discrimination of a huge variety of colours. On the other hand, human visual perception is subject to a plurality of limitations.
For example, the temporal resolution of human visual perception is limited. However, this limitation can be used to achieve a technical benefit. Generating moving pictures is based on this principle. Individual still images acquired one after another with a small temporal displacement are displayed in rapid succession. If the temporal displacement is sufficiently small, i.e. the number of still images acquired in a period of time is sufficiently high, and the speed at which these images are reproduced is chosen accordingly, a viewer gains the impression that he does not watch a discrete set of images, but continuous video content. Thus, recording and reproduction of moving objects with satisfying quality becomes possible by taking advantage of a limitation of the human visual system.
Another limitation of human visual perception concerns the ability to distinguish visual information that stems from objects that are arranged in spatial proximity.
Again, this perceptional shortcoming can be taken advantage of. Colour reproduction in thin film transistor liquid crystal displays (TFT displays) is based on the joint perception of light emitted by three subpixels. Each of these subpixels is a source of light with a different wavelength, i.e. colour, and intensity. Not being able to distinguish the visual information of each subpixel individually, the viewer internally merges the subpixels and obtains the impression of a single pixel. The colour of such a pixel is composed of the colours of the three subpixels.
A similar effect occurs not only on the spatial scale, but on the time scale. Entire images of a certain primary colour but with intensity varying over the image plane can be displayed sequentially. On the premise that the primary colour component images are displayed in rapid succession, the brain merges the primary colour component images, thereby forming a single unitary colour image having the intended colour composition. The primary colour component images are also called fields. Display technologies that are based on this effect are known as field sequential colour displays (FSCDs).
A set of primary colour component images of each primary colour used by the specific display composes a frame. In order to generate unitary images, i.e. frames, with the frequency that a display that employs spatially-modulated colour generation is capable of, the frequency of a FSCD has to be scaled by a factor equal to the number of primary colour components used.
While colour reproduction in, for example, common TFT displays necessitates the provision of a transistor for each subpixel, FSCDs of a TFT type can manage on a single transistor for each pixel. In consequence, FSCDs offer a plurality of advantages.
Production costs decrease vastly. Furthermore, display quality can be enhanced. As displays nowadays often comprise more than one million pixels, and thus more than three millions of transistors, it is likely that not every single transistor has been properly manufactured. Damaged subpixels exhibit behaviour such as emitting light not having the desired intensity or even blocking or letting pass background light permanently. Image quality is thereby deteriorated causing permanent user irritation. Reduction of the number of transistors goes along with a reduction of the probability of the presence of a damaged transistor. Moreover, miniaturization of FSCDs can be achieved easily. With the need of only one transistor for each pixel, pixel density can be increased significantly. In consequence, high resolution displays covering a small area can be realised. This is especially beneficial for mobile devices. Having to control merely one transistor per pixel, power consumption of TFT FSCDs is lower than power consumption of displays that employ spatially-modulated colour generation.
In addition, TFT FSCDs allow for a high pixel aperture ratio, i.e. a high ratio between the area of a pixel that is light-transmissive and the pixel area that is opaque because electronic elements, e.g. signal bus wiring, block the light path. A high aperture ratio permits the use of a less powerful backlight source and thus yields decreased energy consumption.
Displays that employ spatially-modulated colour generation often use filters to make the subpixels emit light of a desired primary colour and thus exhibit reduced energy efficiency. Optical filters selectively transmit light having a particular range of wavelengths, i.e. colour of light, while blocking the remainder. The energy of the blocked spectral portion of the incident light does not contribute to image generation and is therefore not of use.
If a FSCD uses a plurality of light sources which emit light having different colours, there is no need for employing filters to obtain different primary colour component images. Hence, energy consumption is reduced. This is not only environmentally sound, but also increases the operating time of battery-powered devices, in particular mobile devices. However, other approaches to primary colour component image generation exist.
The human visual system allows depth perception. Objects can be visually located in the three dimensional space. A viewer's left and right eye perceive slightly different images when looking at an object due to the eyes' horizontal separation. This is called binocular parallax and results in a binocular disparity. The binocular disparity enables the brain to extract depth information from the two images seen by the left and right eye, respectively.
It is obvious that depth perception constitutes a valuable feature of the human visual system. Thus, a variety of devices for both acquiring and reproducing images that enable a viewer to perceive depth have been developed.
The mode of operation of display technologies for application in this field comprises generating two slightly different images, i.e. stereoscopic images with a suitable disparity, intended for a user's first and second eye, respectively. The first eye can be the user's left eye and the second eye can be the user's right eye or the other way round.
To ensure that each of these images actually reaches the eye it is intended for, a plurality of technical approaches exists. A straightforward manner to realise a device having the capabilities described above, is to use two separate display units that are geometrically arranged so that the shown image content of a first display unit is visible for a user's first eye while the content shown on the second display unit is only visible for the user's second eye. A common technology is to integrate the two separate display units in a head-mounted display (HMD) so that a display unit is arranged opposite to each eye. Thus, stereoscopic image reproduction becomes possible by using comparatively simple means.
Due to the advantages set forth above and other advantages, it is desirable to employ FSCDs within the field of stereoscopic image generation.
However, FSCDs suffer from a major drawback. Whenever the eyes of an observer of an image shown by a FSCD move with respect to the image, successive primary colour component images of a frame do not strike the retina at the same location. In consequence, a different set of cones is involved in the visual perception process. Thus, the brain does not merge the primary colour component images properly. As a result, colour breakup artefacts (CBUs) occur. They emerge as rainbow-like areas located at the edges of image objects. For example, an observer is likely to track an object shown on a display by following its movement with his eyes. As the primary colour component images are shown at the same position on the display, CBUs become visible.
Of course, CBUs do not only originate in the movement of the observer's eyes with respect to an image, but also in the movement of the image with respect to the observer's eyes. For example, a rotating public FSCD used for advertising purposes may cause this effect.
CPUs can be avoided by using high image refresh frequencies. Increasing the refresh frequency corresponds to a shorter duration of the period a primary colour component image is shown. Thus, the distance the eye can move within this period is decreased and therefore the observer does not perceive CBUs. Yet, achieving sufficiently high frequencies constitutes a serious technical challenge.